The present invention relates to a method for applying diversity reception in a mobile communication system in which signals of a specific mobile station are received via at least two separate base stations, the base stations being arranged to forward the signals they receive to at least two separate base station controllers. The invention further relates to a mobile communication system comprising at least one mobile switching centre, at least a first and a second base station controller which have a data transmission connection with each other and which are arranged to simultaneously receive signals from a specific mobile station, via a first and a second base station, respectively, which have a data transmission connection with them. The invention also relates to a base station controller of a mobile communication system, the base station controller comprising means for receiving signals from a mobile station, via a base station having a data transmission connection with the base station controller.
Previously known is, from CDMA (Code Division Multiple Access) systems, for example, a procedure known as xe2x80x98soft handoverxe2x80x99. In soft handover, diversity combining is applied to receive signals from a specific mobile station via two separate base stations. The mobile station thus moves from one cell to another without any abrupt handover. Instead, handover from one base station to another proceeds gradually as the mobile station approaches a new base station, the new base station thus receiving the signals from the mobile station stronger and stronger. A drawback of this prior art solution is that it unnecessarily loads higher elements of the network hierarchy. The main reason for this is that, in known solutions, the above mentioned higher network elements of the hierarchy, in particular, take care of diversity combining.
An object of the present invention is to eliminate the above drawbacks and to provide a solution for carrying out diversity reception in a mobile network as efficiently and extensively as possible, while causing a minimum load on the network resources. This object is achieved with a method of the invention, characterized by arranging a first base station controller to forward the signals received from a base station to a second base station controller, which is arranged to apply diversity combining to forward the signals received from a base station and from the first base station controller further to a mobile switching centre, whereby a handover involving said mobile station comprises the following:
arranging the second base station controller to forward the signals received from the base station to the first base station controller;
setting up a connection from the first base station controller to the mobile switching centre; and
arranging the first base station controller to apply diversity combining to forward the signals received from the base station and from the second base station controller further to the mobile switching centre.
The invention further relates to a mobile communication system to which the method of the invention can be applied. The mobile communication system of the invention is characterized in that the first base station controller is arranged to forward the signals received from the first base station to the second base station controller, which is arranged to apply diversity combining to forward the signals received from the second base station and from the first base station controller further to the mobile switching centre, and that, in connection with handover, the base station controllers are arranged to exchange functions, the second base station controller, which earlier applied diversity combining, thereafter forwarding the signals received from the second base station to the first base station controller, which thereafter forwards the signals received from the first base station and from the second base station controller further to the mobile switching centre.
The invention also relates to a base station controller which can be utilised in the mobile communication system of the invention. The base station controller of the invention is characterized in that the base station controller can be set at least at two alternative modes; in a first mode, combining means of the base station controller are arranged to apply diversity combining to forward the signals received from the base station and from a second base station controller to a mobile switching centre; and, in a second mode, the base station controller is arranged to forward the signals received from said mobile station to said second base station controller.
The invention is based on the idea that a solution which does not unnecessarily occupy the resources available to the system can be obtained by arranging base station controllers to carry out diversity combining in such a way that one of two base station controllers carries out the combining and the data transfer to the mobile switching centre, the other one merely forwarding the signals it receives to the base station controller carrying out the combining. In connection with handover, for example, resources can be thus be saved by arranging the base station controllers to exchange functions: the base station controller that previously carried out the combining can abandon its connection to the mobile switching centre and start to forward the signals it receives to the second base station controller which, in turn, starts to carry out diversity combining and to forward signals to the mobile switching centre. This allows the advantages of soft handover to be made available, without the handover in itself occupying the system any more than a conventional handover applied in the GSM system (Global System for Mobile communications), for example.
Consequently, the most significant advantages the invention offers are that the network elements highest in the network hierarchy are not unnecessarily loaded and that the data transmission connection between the base station controllers is not unnecessarily loaded, because the combined signals can always be forwarded via the shortest route to the mobile switching centre, in other words, direct from the base station controller that carried out the diversity combining.
The preferred embodiments of the method, mobile communication system and base station controller are disclosed in the appended dependent claims 2, 3, 5, 6 and 8.